1. Technical Field
The invention relates to a mobile communication device, and more particularly, to an impedance matching method for the mobile communication device.
2. Background
Impedance matching is important to any mobile communication device which uses an antenna for transmitting signals. When a power amplifier of the mobile communication device transmits a radio frequency (RF) signal to the antenna, if the antenna shows bad impedance matching, a part of forward power will be reflected by the antenna, which substantially reduces transmit power of the antenna. Different types of antennas have different impedances. The antenna impedance changes when the environment changes, e.g., when a user's hand is close to the mobile communication device or when the mobile communication device is close to a table surface. Moreover, the manner of change varies among different types of antennas. These facts add more complexities to the issue of impedance matching.
Most conventional mobile communication devices use a tunable matching circuit to make the impedance encountered by the RF signal equal to a load-pull impedance corresponding to the power amplifier. The load-pull impedance is the best impedance among the impedances possibly encountered by the RF signal being outputted from the power amplifier, and that is, the load-pull impedance is capable of reducing the reflection of the RF signal by the antenna to a minimum, even to zero.
Such tunable matching circuit has one or more tunable capacitors or tunable inductors, and the impedance of each tunable capacitor and tunable inductor is tunable, thereby tuning the aforementioned impedance encountered by the RF signal. Each tunable capacitor and tunable inductor has a tunable range covering multiple setting values. A traditional approach is trial and error, i.e. to try each setting value one by one until the tunable matching circuit is able to provide the load-pull impedance. This approach may take longer to complete, thus indirectly degrading performance of the antenna.